Volumetric Shading in Cycles
One of the standards for faking volumetrics is using billboard particles. Back in Blender v2.49, I used to use them all the time. I know that Weta digital used billboard particles for smoke in the LoTR movies. I started a thread on blenderartists.org about it. I did a series of video tutorials about it that are listed in the thread. They aren't the greatest video tutorials, but you may find them useful.

Basically, each particle in your system is rendered as a flat, transparent, circular texture. With enough particles, it will look like smoke/cloud. You can even use textured spheres as your particles if a strictly 3D effect is necessary. I talk about using spheres in the third video.

Alternatively, you can render all your volumetrics in a separate layer using BlenderRender. Then, you can composite them into your cycles scene later.

Here is a video with examples